Pacemaker system with porous electrode and residual charge or after-potential reduction

ABSTRACT

A cardiac pacemaker system is provided which includes a stimulation electrode adapted for being anchored in the heart. An output capacitor is coupled to the stimulation electrode. A first circuit coupled to the output capacitor generates stimulation pulses. A second circuit coupled to the output capacitor generates an autoshort pulse following each stimulation pulse to reduce a residual charge of the output capacitor for eliminating an after potential following a stimulation pulse by the stimulation electrode. A third circuit coupled to the output capacitor acquires an evoked pulse of the heart from an electrical signal picked up by the stimulation electrode. The stimulation electrode includes a porous surface coating made of an inert material and has an active surface that is substantially larger than a surface of the basic geometric form of the stimulation electrode. The second circuit includes circuit means for changing the time duration of the autoshort pulses as a function of the acquisition of the evoked pulses, with the time duration of the autoshort pulses being limited to 70 ms.

BACKGROUND OF THE INVENTION

The invention relates to a cardiac pacemaker system of the typeincluding a stimulation electrode adapted for being arranged in theheart; an output capacitor coupled to the stimulation electrode; a firstcircuit, coupled to the output capacitor, for generating a pulsefollowing each stimulation pulse for at least one of reducing a residualcharge of the output capacitor and eliminating an afterpotentialfollowing a stimulation pulse by the stimulation electrode; and a thirdcircuit, for acquiring an evoked heart action from an electrical signalpicked up by an electrode arranged in the heart.

For a long time, it has been a goal in the development of artificialcardiac pacemakers to verify the success of a heart stimulation throughthe measurement and evaluation of signals, which can be picked up in theheart on the basis of the evoked heart action via an electrode which isinstalled in the heart--and preferably the stimulation electrode itself.

The pickup of the electrical "response signal" of the heart after astimulation is disturbed by the aftereffects of the stimulation pulse,which are caused by the polarization of the stimulated tissue, which canbe reduced only when the recharging of the output (coupling) capacitorconnected to the stimulation pulse is also eliminated.

This follows from the fact that the evoked potential, which indicatesthe success of the stimulation and is present at the heart approximatelyup to 300 ms after the stimulation, is superposed by an afterpotentialin the order of magnitude of more than 10 mV. The afterpotential, whichdisturbs the effectiveness for recognition of the evoked potential, iscaused by the effect of the stimulation electrode as an electrochemicalelectrode, from which results a saturation of the detection amplifier.

Apart from various circuits, with which attempts are made to eliminatethe consequences of the afterpotential, an apparatus for the stimulationof the heart is known from EP-B1-0 000 989, wherein the disturbingafterpotential of the stimulation electrode is intended to be reduced inan accelerated manner by means of an additional, transistor-controlledresistor branch, which essentially short-circuits the Helmholtzcapacity. The total time needed for the reduction of the disturbingafterpotential, however, is too long with customary electrodes to makepossible an effective effectiveness recognition under all circumstances.

SUMMARY OF THE INVENTION

Starting from the drawbacks of the prior art, it is the object of theinvention to provide a cardiac pacemaker system of the generic typementioned in the introduction, in which an effective detection of evokedheart signals is possible in an effective manner, also under thedifferent changing operating conditions of a cardiac pacemaker, whichoccur, for example, during the settling in of the electrode.

The above and other objects are accomplished in the context of a cardiacpacemaker system of the type first described above, wherein according tothe invention the stimulation electrode includes a porous surfacecoating made of an inert material and having an active surface that issubstantially larger than a surface of the basic geometric form of thestimulation electrode; and the pacemaker system includes circuit meansfor changing the time duration of the activation of the second circuitas a function of the acquisition of the evoked heart action, with thetime duration of the activation of the second circuit being limited tono more than 70 ms.

The invention includes the finding that when an electrical voltage isapplied to a pacemaker electrode, which is anchored in the heart, twolayers of different charge carriers are formed, which, however, areseparated by a monolayer of hydrogen molecules based on hydrationeffects. In its structure and electrical behavior, this so-calledHelmholtz double layer corresponds to a plate capacitor. If, during thestimulation of the heart, a current flows via this Helmholtz capacity, avoltage is generated there which forms the afterpotential, with thevoltage increasing as the Helmholtz capacity decreases. Theafterpotential is additionally increased through further electrochemicalreactions with charged reaction products taking place at the phaseboundary. Apart from the increase of the Helmholtz capacity, a reductionof the stimulation pulse amplitude, above all, is important for reducingthe afterpotential so that a definitive effectiveness recognition can becarried out with the same electrode. In addition, a reduction of theamplitude of the stimulation pulse contributes in an advantageous mannerto increasing the service life of the pacemaker's current supply source.

The selection of the measures according to the invention can thus, onthe one hand, reduce the stimulation pulse amplitude so that theafterpotential as a whole becomes lower. Moreover, the reduction of theafterpotential is accelerated so that the afterpotential is reducible ina defined manner within a predetermined period of time.

This reduction can take place by means of an active counterpulse or alsothrough passive means at the output of the stimulation circuit, as isknown from the prior art that was mentioned (autoshort).

According to advantageous modifications of the invention, it becomespossible through automatically operating circuit means to automaticallydetermine the time duration of the blocking of the input amplifier forthe evoked pulses and to adapt it to the implantation conditions ortheir temporal change. In this process, an increased stimulus energy isused, which, with certainty, leads to a stimulation.

Additionally, in an advantageous modification, a cardiac pacemakersystem can be provided, which overall only has a low energy requirementbecause of the automatic adjustment of the stimulation amplitude.

It was recognized here that

while a stimulation pulse having an excessive amplitude leads withcertainty to a stimulation of the myocardium, the service life of thepacemaker's current supply source, however, is considerably reducedbecause of the increased energy consumption so that an earlyreimplantation must be carried out,

a sufficiently reliable detection of evoked signals, based on amyocardium stimulation that has taken place, is possible only after asufficient reduction of the afterpotential, which occurs due to thestimulation pulse, if stimulation and detection are carried out with thesame electrode,

the afterpotential may, at most, reach such a value which can be reducedto a negligible level within a period of approximately 30 to 80 ms(autoshort), before an evoked potential has decayed and

the materials of the known electrodes and, in particular, titanium,vanadium, zircon and niobium tend to, at times, show extreme oxidationand that, in case of contact with aqueous electrolytes, this highoxidation tendency leads to the formation of a thin, insulating orsemiconductive oxide layer at the electrode surface, with the oxidelayer representing a capacity C_(ox) connected in series with theHelmholtz capacity C_(H) and thus leading to a slow reduction of thetotal capacity and therewith to the corresponding increase of therespectively required stimulation energy.

The pulse control of the control system according to the invention isconfigured both for the automatic determination of the width of theautoshort pulses, which is necessary for the detection of evokedpotentials, and for maintaining a minimum amplitude of the stimulationpulses, which exceeds the stimulus threshold of the myocardium at thedetermined necessary width of the autoshort pulses, and it is providedwith the electrical means necessary for this purpose. These essentiallycomprise a controllable autoshort pulse generator, a generator for thegeneration of amplitude-controlled stimulation pulses controlled by agate circuit at a predetermined pulse repetition frequency and devicesfor the detection of the potentials evoked by the stimulation pulses asa function of the width of the autoshort pulses. The automatic settingof the autoshort time is among the most essential advantages of thepulse control.

The operation of the pulse control circuit represented here takes placein two different operating conditions, "alignment" and "continuousoperation". According to the preferred embodiment of the invention, apulse generator is provided for the generation of the autoshort pulses,in which a variation of the pulse width in the "alignment" operatingcondition is carried out in a scanning manner by a ramp generator. Inthis process, the stimulation pulses are kept constant with regard totheir amplitude through a corresponding setting in the pulse amplitudecontrol of the stimulation pulse generator, at a level which is abovethe stimulus threshold of the myocardium and at which an evokedpotential is released with certainty. The correspondingly detected,pulse-shaped signals are fed to a memory after sufficient amplification.

The memory is configured in a matrix fashion or array fashion and isaddressed by the above-mentioned ramp generator such that an allocationof the memory locations to the evoked signals takes place as a functionof the respective autoshort pulses of a certain width.

An evaluation unit downstream of the matrix memory determines the mosteffective detection of the evoked potentials with respect to the pulsewidth of the autoshort pulses. This autoshort time is fixed in thegenerator for the autoshort pulses and sets as a self-adjusted value thewidth of the autoshort pulses for the "continuous operation" operatingcondition of the pulse control following the "alignment" operatingcondition. For the change-over of the operating conditions, a cyclicaltimer switch is provided by means of which the ramp generator, thegenerator for the autoshort pulses and the amplitude control stage ofthe stimulation pulses can be correspondingly switched on or off. Duringthe "continuous operation" of the pulse control, a gate circuit, whichis provided at the input of the amplitude control stage for thestimulation pulses, is activated by the timer switch and the detectionpulses of the evoked potentials.

Each pulse that corresponds to a detected potential leads to a reductionof the amplitude of the stimulation pulses by a certain amount. If,after a number of stimulations, the stimulation pulse remains below thestimulus threshold, an evoked potential can no longer be picked up. Acorresponding output signal at a gate circuit leads to an increase ofthe amplitude of the stimulation pulses in the downstream amplitudecontrol stage to the value that was last applied successfully. Thisaccomplishes that the stimulation pulse, which follows the missingdetection of an evoked potential, leads with certainty to a renewedstimulation of the myocardium and that a "falling-out-of-step" of thesynchronization of the total system is prevented.

It is evident that, instead of the stimulation amplitude, also the pulsewidth or another value that determines the stimulus energy can bechanged.

It is also particularly advantageous if the afterpotential iscompensated through an active counterpulse, because the electrode usedin the cardiac pacemaker system according to the invention can also beoperated anodically, without an oxide layer impairing the stimulationthreshold.

According to an advantageous modification of the invention, theamplitude increase in case of a missing detection of an evoked potentialis a multiple of the value of the amplitude decrease when a detectiontook place. This is accomplished in a simple manner by means of adivider circuit, which provides the output signals of the gate circuitfor the amplitude reduction with this factor.

According to another advantageous embodiment of the invention, a changeof the switching conditions of the timer switch takes place in timeintervals of equal length, which cyclically repeat themselves, in orderto regularly carry out a control of the selected autoshort time. It hasproven advantageous to again carry out an "alignment" after apredetermined number of lowering cycles of the amplitude of thestimulation pulses until a potential detection first fails to appear soas to adjust the autoshort time, if necessary, to a possible change ofthe ability of the myocardium to be stimulated.

The function of the pulse control according to the invention is onlyguaranteed for autoshort times in the range of 50 ms if the constructiveconfiguration of the stimulation electrode accomplishes that only arelatively low afterpotential is built up following the stimulationpulse.

According to the preferred embodiment of the invention, the stimulationelectrode is provided with a porous surface coating made of an inertmaterial, with the active surface of the coating being considerablylarger than the surface that results from the geometric shape of theelectrode. Because of the fractal spatial geometry, the active surfaceis so large that the energy required for the stimulation can be set to aminimum value. Thus, because of the electrodes' large relative surface,a successful stimulation with low energy is possible, in principle, forthe conventional coated, porous electrodes. It was now recognized thatthe Helmholtz capacity is reduced due to the oxidation tendency, whichleads to an increase in the electrode impedance. The reason why theinfluence, which is thus generated, on the electrode properties in thecourse of the implantation time is so serious is that the deteriorationof the electrode properties has consequences which, in turn, contributeto the fact that the stimulation properties are also influencedadversely.

Thus, for a deteriorating electrode, a greater pulse energy is necessaryso that, for the effectiveness recognition, a counterpulse with agreater energy requirement is also necessary which, in turn, againcontributes to the deterioration of the electrode properties. Since thepulse energy and the counterpulses necessary for the effectivenessrecognition are set to values that have to have validity over the totalimplantation time of the pacemaker, the deterioration of the operatingconditions ultimately is essentially based on measures, which areactually intended to counteract the deteriorated operating conditions.

The long-term-stable, biocompatible surface coating of the stimulationelectrode according to the invention is made of a material whoseoxidation tendency is very low, with the coating being applied on theelectrode using vacuum technology, preferably by using an inertmaterial, namely a nitride, carbide, carbonitride or a pure element orcertain alloys from the group gold, silver, platinum, iridium, titaniumor carbon. Owing to the fractal spatial geometry of a surface layerapplied in this manner, its active surface is very large so that theamount of energy needed for the stimulation can be kept extremely low.

The afterpotential of a stimulation electrode made of titanium, which isprovided with a sputtered iridium nitride layer or titanium nitridelayer by means of the reactive cathode sputtering, is smaller by up tosix times (from approximately 600 mV to approximately 100 mV) than theafterpotential of a bare stimulation electrode made of titanium. Owingto this significant reduction of the afterpotential, the recognition ofthe intracardiac ECG is possible not only in the conventional manner bymeans of an amplifier and a triggering device, but an operativeeffectiveness recognition can be applied, which can do withoutcounterpulse and autoshort times for the reduction of the afterpotentialin the magnitude of 50 ms.

Advantageous modifications of the invention are described below ingreater detail in conjunction with the accompanying Figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the preferred embodiment of the invention.

FIG. 2 is an overview diagram containing the most important influencingquantities for the pulse control in a schematic representation.

FIG. 3 is an amplitude-time diagram, shown schematically, for thegenerated stimulation pulses and the detected evocation potentials.

FIG. 4 is an embodiment of a stimulation electrode representedschematically in side view.

FIG. 5 is an enlarged representation of detail A in FIG. 4 in asectional view.

FIG. 6 is a diagram to compare the impedance of the embodiment of theelectrode with the impedance of corresponding electrodes known fromprior art having the same geometric dimensions.

FIG. 7 is a representation of the afterpotential as a function of theautoshort time in dependence of the surface configuration of theelectrode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The means for the pulse generation of the control system according tothe invention are schematically illustrated in FIG. 1. The stimulationelectrode, which is part of the control system, is connected to theoutput capacitor 1 and is not shown in the drawing.

The electrode is provided with a pulse generator 4 for stimulationpulses that are released in the direction 2 onto the stimulationelectrode.

A time control stage 6 determines the point in time of the release ofstimulation pulses and, in this case, corresponds to a fixed frequencypacemaker. The schematic circuit diagram is also usable for otherpacemaker circuits, where merely additional control lines must beprovided, through which, for example, in a demand pacemaker, stimulationis prevented through the release of stimulation pulses in case ofsignals stemming from heart actions that come in before the end of theso-called escape interval. With an amplitude control stage 5, theamplitude (or the energy) of the stimulation pulses can be raised ("+")or lowered ("-") via additional inputs.

In addition, a pulse generator 15 is provided for the generation ofautoshort pulses via the final pulse generator stage 4. Via a galvanicconnection or an active counterpulse, the potential of the innerconnection of the capacitor 1 is returned in this process to the initialstate prior to the last stimulation pulse so that, by way of the chargeshift generated, the afterpotential at the electrode is counteracted.The time duration of the pulse for eliminating the aftereffects of thestimulation pulse can be set via a corresponding input of the pulsegenerator 15.

Via an amplifier 11, signals that are generated by the heart are pickedup, with the amplifier being switched so as to be insensitive byswitching means, that are not shown, when a stimulation pulse occurs. Anevoked event is retained in a memory 12. In order to be able to optimizethe time duration of the autoshort pulse, a signal indicating an evokedevent is retained in allocation to the duration of the correspondingautoshort pulse.

The "alignment" operating mode is set by means of a control switch 17while, otherwise, the circuit is in the "continuous operation" operatingmode.

During the "alignment" operating condition, the optimum autoshort timeis determined, which is then maintained in the "continuous operation"position. For this purpose, a pulse amplitude is predetermined by thetimer switch 17 via the control line 24 in the amplitude control 5 at aconstant frequency (time control 6), at which pulse amplitude an evokedpotential at the myocardium is generated with certainty. Simultaneously,the timer switch 17 activates, via the control line 26, a ramp generator16, which is connected to the pulse generator 15 via a change-overswitch 14 to vary the width of the autoshort pulses in in a scanningmanner. The AND gate 9 is blocked, also controlled by way of the timerswitch 17 via the line 27 and a negator 10.

A picked up evoked potential or a corresponding signal 3 indicating thiscondition is fed to an amplifier 11 via the connecting line 32 andacquired in a matrix memory 12. The allocation of the individual memorylocations takes place in dependence of the time function of the rampgenerator 16 so that to each pulse width a signal can be allocated,which indicates the pickup of an evoked potential. An evaluation circuit13 determines the most favorable autoshort time for the detection of theevoked potentials 3. In this process, a mean value of all pulsedurations of the autoshort pulse, at which an evoked potential could bepicked up, is selected so as to have a certain amount of certainty withrespect to the change of the signal pickup conditions in the course ofthe operating time of the pacemaker.

Subsequently, the switch 17 is reset to the "continuous operation"operating condition, during which process the mean value of theautoshort time, at which an evoked potential could be picked up, isretained in the pulse generator 15 via the change-over switch 14 and theline 25, and the AND gate 9 is released via the negator 10. Afterwards,the stimulation amplitude is again lowered to its normal value.

It is now possible with the evoked potentials, which can be recognizedreliably because of the alignment that was carried out, to set thestimulation energy (stimulation amplitude) during the operation withthreshold control via an effectiveness recognition in such a way thatthe stimulation threshold is reliably exceeded without a prematureexhaustion of the energy source occurring because of an excessivestimulation energy.

Each detection of an evoked potential 3 generates a pulse via the ANDelement 9 at the divider 7, which pulse decreases the amplitude of thenext stimulation pulse 2 by a certain amount. This step-by-stepamplitude reduction takes place until no evoked potential is detected atthe predetermined autoshort time. The level change at the output of theAND gate 9 switches the negator 8 and then effects a raising of thestimulation amplitude up to a preceding value at which a stimulationtook place reliably. Via the divider 7, an amplitude decrease only takesplace at every nth (here 20. This value only represents an example,because, in practice, the stimulus threshold will stabilize in the longterm so that divider ratios of several thousand will be practicable.)successful stimulation pulse--but a raising immediately following everyfailed stimulation. Thus, the stimulation pulses are always providedwith a stimulus energy, in particular, amplitude, which is only slightlyabove the stimulus threshold, respectively leading to a heartstimulation with great certainty.

In order to acquire possible changes in the transmission ratios of themyocardium, it is of particular advantage after a "continuous operation"phase of the pulse control to again determine the autoshort time, whichis optimal for the stimulation and detection of the heart activity, in arepetition of the "alignment." It has proven advantageous to carry out afurther "alignment" for the amplitude of the stimulation pulses after a"continuous operation" with, for example, m-cycles. In addition, it ispossible to adjust the change-over cycle of the switch 17 to thepatient-specific conditions.

The schematically illustrated diagram of FIG. 2 shows, on the time axis,the possibility of picking up evoked potentials in the heart as afunction of the variation of the autoshort time and of the stimulationamplitude.

Evoked signals can only be picked up if a stimulation pulse iseffective, which means that the pulse has exceeded a predeterminedthreshold energy, as it is indicated by the horizontal line 21. Inaddition, the possibility of the pickup of evoked signals is furtherlimited by the decay of the evoked potential, which is indicated by line19 as limit for the decay of the stimulation effect (afterpotential).The line has a slight gradient, because, with a higher stimulationamplitude, the (disturbing) afterpotential also increases or theduration of its decay becomes longer. The point in time 20 forms thattime mark after which an evoked potential has decayed to such a lowlevel that its detection is no longer possible or the event of interesthas passed. With the measures according to the invention, a time rangefor the measures to eliminate the afterpotential is set during anautomatic adjustment of the duration of the autoshort time, this timerange being within the effective range. Between the limit valuesgenerated by the lines 19 and 20, in particular, a mean value is set.The coating of the stimulation electrode according to the inventionmakes possible a lowering of the afterpotential, which disturbs thedetection of the evoked potentials, at an autoshort time of 50 ms to avalue of almost 0 mV (compare FIG. 7).

FIG. 3 shows the amplitude-time-diagram of the stimulation pulses 24 inrelation to detectable evoked potentials 25 during the "continuousoperation" operating condition of the pulse control. After eachstimulation pulse 24, for which an evoked potential 25 is detected afterthe autoshort time T=t_(E) -t_(S), a step-by-step amplitude reductiontakes place via the pulse amplitude control (compare position 5 in FIG.1). If the detection limit with the stimulus threshold 21 is reached orif a slight shortfall occurs, the resulting change in potential at theoutput of the gate circuit (comprising elements 7, 8, 9, 10 in FIG. 1)effects a renewed increase of the amplitude of the subsequentstimulation pulse 24. The amplitude jump occurs, in particular, to theamplitude value at which a successful stimulation has last taken place.

In order to keep the number of shortfalls of the stimulus threshold, atwhich effective stimulation does not occur, as low as possible, alowering is only carried out at every nth stimulation pulse inadvantageous embodiments of the invention, with a raising immediatelyfollowing every threshold shortfall.

The stimulation electrode 100, illustrated in FIG. 4 in a schematic sideview, is a unipolar nap electrode having a head that is provided with acylinder-shaped basic body 126 made of titanium. The cylinder-shapedbasic body 126 is provided with a surface coating 127 consisting of aninert material iridium nitride (IrN), which is applied to thecylinder-shaped basic body 126 of the titanium electrode by means ofcathode sputtering. The electrode is provided with a coiled electricallyconductive lead 131, which is provided with an electrically insulatingsheathing 130 made of silicon. This silicon sheathing is shown to betransparent in the drawing. Formed to the silicon sheathing are flexiblefastening elements 129 oriented rearward, which serve to anchor theelectrode in the heart, with the surface of the basic body being kept incontact with the inner heart surface.

By means of a hollow-cylindrical shoulder 128, the basic body 126 isslid over the lead 131 and fastened there, with this shoulder beingshown in sectional view in the drawing.

FIG. 5 is an enlarged view of a section (detail A in FIG. 4) of theactive surface. As is evident from the illustration, the fractal spatialgeometry (enlarged not to scale) of the coating 127, grown in themicroscopic range in a stem-like manner, accomplishes an essentialenlargement of the active surface. The surface enlargement achieved isin the range of more than 1000.

As can be seen from FIG. 6, which shows a comparison of the impedancecurves of stimulation electrodes having different surface coatings, anelectrode which is coated with iridium nitride has the lowest phaseboundary impedance for picking up heart signals for which thelow-frequency range is particularly important, especially in the regionwhere the signals are weak, as compared to titanium or titanium nitridewhich are recognized state of the art electrode surface materials. Thedifferences determined are particularly essential in their consequencesfor the reason that the amplitude of the picked up signal is related ina square function to the internal resistance of the signal source.

FIG. 7 illustrates the measurement results, which show theafterpotential generated by the stimulation as a function of theautoshort time T in dependence of the configuration of the stimulationelectrode. Since the evoked potential indicating the success of amyocardium stimulation can be found in a time range of 50 to 300 msafter the stimulation, its detection can occur without disturbance witha titanium-nitride-coated stimulation electrode at autoshort times of 50ms, whereas the evoked potential is "covered" by afterpotentials in themagnitude of 10 mV in uncoated platinum electrodes. This also makes thedetection of the evoked potentials at a point in time after 50 msconsiderably more difficult and is not possible with uncoatedstimulation electrodes, since the amplitude of the evoked potentialreduces itself very quickly after generation and drops below the levelof the remaining afterpotential.

The invention is not limited in its implementation to the preferredembodiment described above. On the contrary, a number of variants areconceivable which utilize the described solution, also if theembodiments are, in principle, of a different type.

We claim:
 1. A cardiac pacemaker system including: a stimulationelectrode adapted for being arranged in the heart; an output capacitorcoupled to the stimulation electrode; a stimulation circuit, coupled tothe output capacitor, for generating stimulation pulses; apulse-generating circuit, coupled to the output capacitor, and beingactivated for generating a pulse following each stimulation pulse for atleast one of reducing a residual charge of the output capacitor andeliminating an afterpotential following a stimulation pulse by thestimulation electrode; and an evoked response circuit, coupled to acontrol input of the pulse generating circuit, for acquiring an evokedheart action from an electrical signal picked up by an electrodeinstalled in the heart, the improvement wherein:the stimulationelectrode includes a porous surface coating made of an inert materialand having an active surface that is substantially larger than a surfaceof the basic geometric form of an uncoated stimulation electrode; andthe pacemaker system includes circuit means for changing a time durationof the activation of the pulse generating circuit as a function of theacquisition of the evoked pulses, with the time duration of theactivation being limited to no longer than 70 ms.
 2. The cardiacpacemaker system of claim 1, wherein the time duration of the activationof the pulse generating circuit is no longer than 50 ms.
 3. The cardiacpacemaker system of claim 1, wherein the circuit means for changing thetime duration of the activation of the pulse generating circuitautomatically sets the time duration during an alignment period inwhich, with an amplitude of the stimulation pulses at a raised level,the time duration of the activation of the pulse generating circuit issuccessively varied between individual stimulation pulses until anevoked heart action is acquired by the evoked response circuit.
 4. Thecardiac pacemaker system of claim 3, wherein the circuit means forchanging the time duration of the activation of the pulse generatingcircuit reduces the time duration of the activation, starting from amaximum value.
 5. The cardiac pacemaker system of claim 3, wherein thecircuit means for changing the time duration of the activation of thepulse generating circuit includes means for raising the amplitude of thestimulation pulses, means for varying the time duration of theactivation of the pulse generating circuit and means for storing theacquisition of each evoked heart action in allocation to a respectivetime duration of the activation of the pulse generating circuit.
 6. Thecardiac pacemaker system of claim 3, and further including meansconnected to the stimulation circuit for changing the amplitude of thestimulation pulses to a level which assures reliable acquisition of anevoked heart action by the evoked response circuit after the automaticsetting of the time duration.
 7. The cardiac pacemaker system of claim6, wherein the means for changing the amplitude of the stimulationpulses includes a regulating circuit for reducing the amplitude of thestimulation pulses by a first predetermined amplitude amount if anevoked heart action is detected by the evoked response circuit and forincreasing the amplitude of the stimulation pulses by a secondpredetermined amplitude amount if no evoked heart action is detected bythe evoked circuit, wherein the regulating circuit is operative when thecircuit means for changing the time duration of the activation of thepulse generating circuit is inoperative.
 8. The cardiac pacemaker systemof claim 7, wherein the first predetermined amplitude amount is smallerthan the second predetermined amplitude amount.
 9. The cardiac pacemakersystem of claim 7, wherein the means for changing the amplitude of thestimulation pulses is operative for reducing the amplitude of thestimulation pulses by the first predetermined amount at every nthdetection of an evoked heart action by the evoked response circuit. 10.The cardiac pacemaker system of claim 1, and further including anaddressable memory connected to the evoked response circuit for storingthe acquisitions of evoked heart actions.
 11. The cardiac pacemakersystem of claim 5, wherein the circuit means for changing the timeduration of the activation of the pulse generating circuit includes aramp generator for producing a ramp output signal, and the means forstoring the acquisition of the evoked heart actions comprises anaddressable memory which allocates the acquisition of evoked heartactions to memory locations as a function of the ramp output signal. 12.The cardiac pacemaker system of claim 11, wherein the circuit means forchanging the time duration of the activation of the pulse generatingcircuit includes an evaluation unit coupled to the addressable memoryfor determining a mean time duration for the activation of the pulsegenerating circuit at which evoked heart actions are acquired and fixingthe time duration of the activation of the pulse generating circuit tothe mean duration during a continuous operating mode.
 13. The cardiacpacemaker system of claim 12, wherein the circuit means for changing thetime duration of the activation of the pulse generating circuit includesa change-over switch coupled to the pulse generating circuit forconnecting the pulse generating circuit in the alignment period with theramp generator and for coupling the pulse generating circuit in thecontinuous operation mode with the evaluation unit.
 14. The cardiacpacemaker system of claim 1, wherein the active surface of thestimulation electrode has a fractal spatial geometry and is larger by afactor of at least one thousand than the surface comprising the basicgeometric form of the stimulation electrode.
 15. The cardiac pacemakersystem of claim 1, wherein the inert material is selected from a groupcomprising a nitride, carbide, carbonitride and a pure element or alloyselected from the group comprising gold, silver, titanium, iridium,platinum and carbon.
 16. The cardiac pacemaker system of claim 1,wherein the porous surface coating is applied to the stimulationelectrode by a thin-film technology.
 17. The cardiac pacemaker system ofclaim 16, wherein the thin-film technology comprises one of reactivecathode sputtering and ion plating.
 18. The cardiac pacemaker system ofclaim 1, wherein the stimulation electrode has a basic body comprised oftitanium on which the porous surface coating is applied.
 19. The cardiacpacemaker system of claim 1, wherein the electrode for picking up anelectrical signal of an evoked heart action comprises the stimulationelectrode.